pH-Control of the Photocatalytic Degradation Mechanism of

ARTICLE pubs.acs.org/JPCC

pH-Control of the Photocatalytic Degradation Mechanism of Rhodamine B over Pb3Nb4O13 Oliver Merka,*,† Viktor Yarovyi,† Detlef W. Bahnemann,‡ and Michael Wark† † ‡

Institut f€ur Physikalische Chemie und Elektrochemie & Zentrum f€ur Festk€orperchemie und neue Materialien (ZFM) Institut f€ur Technische Chemie, Leibniz Universit€at Hannover, Callinstrasse 3A, 30167 Hannover, Germany ABSTRACT: Lead niobate Pb3Nb4O13 was synthesized via chemical coprecipitation method and additionally deposited on silica Aerosil nanoparticles. The thus synthesized catalysts were characterized by X-ray diffraction, UVvis reflectance spectroscopy, light scattering, and electron microscopy. Several samples were tested for the photocatalytic rhodamine B degradation at different pH values focusing on a determination of the degradation mechanism, that is, either the direct oxidative mineralization to carbon dioxide and water or the deethylation yielding the 4-fold deethylated rhodamine 110. For unsupported lead niobates, the degradation rates increase with decreasing pH value, while pH variation has no influence on degradation rates of Aerosil supported samples. An adsorption model is introduced revealing that significant deethylation only proceeds provided that special interactions between the catalyst surface and the dye are existent. As the surface charge and the structure of the dye are both functions of the proton concentration, adjusting the pH value controls the deethylation process.

’ INTRODUCTION The interest in semiconductor photocatalysts has grown over the past decades, as many environmental problems are expected to be solved by photocatalysis.1 A photocatalytic reaction can be described by three main reaction steps. First, the semiconductor absorbs a photon with suitable energy to excite an electron from the valence band into the conduction band, that is, band gap excitation. Thus, an electron hole pair is generated. Subsequently, the generated charge carriers diffuse to the particle surface, while recombination can occur at lattice defects. Finally, electrons and holes react as reducing and oxidizing reagents, respectively. Usually anatase, the most active modification of TiO2, is used as photocatalyst, exhibiting a band gap energy of 3.2 eV. Therefore, this material can only be excited by photons with wavelength shorter than 385 nm, which corresponds to merely 4% of the available solar energy. To decrease the band gap energy to also utilize the visible part of the solar spectrum for photocatalytic processes, TiO2 can be doped with nitrogen2 or carbon.3 On the other hand, some binary compounds such as CdS and Bi2O3 exhibit visible light activity inherently, with band gap energies of 2.54 and 2.85 eV, respectively. In recent years, ternary oxides became the focus of attention, as the variety of possible elemental combinations was promising a plenitude of new visible light driven photocatalysts. Especially ternary oxides with a corner-sharing octahedral network of nd0-transition metals, for example, perovskites and pyrochlores, showed high photocatalytic activity.6 Among diverse structures, for example, the pyrochlore PbBi2Nb2O97 and preferentially perovskites such as CeTiO4,8 Bi2WO6,9 and InVO410 were tested for their photocatalytic activity in various degradation reactions. There are two r 2011 American Chemical Society

main fields of application. On the one hand, energy conversion can be achieved by photocatalytic water splitting into oxygen and hydrogen.11 For this, the reducing potential of the electrons as well as the oxidizing potential of the holes are mandatory. On the other hand, many organic compounds, especially pollutants, can be fully decomposed by photocatalysis.12 These reactions can be divided into two classes. Dye degradation reactions are of practical relevance for water cleaning in the textile and photographic industries. Extensive studies have been performed, for example, on methylene blue13 and rhodamine B.14 Photocatalytic dye degradation reactions often proceed via photocatalytically induced radical reactions leading to a variety of possible reaction products and intermediates. Therefore, reaction mechanisms and especially photonic efficiencies are difficult to determine. Moreover, dyes can be excited by visible light irradiation leading to photochemical reactions or indirect photocatalytical processes such as the deethylation process. Consequently, dye degradation reactions are unfavored model reactions for comparing activities of different catalysts. Instead, easy model reactions were developed, for example, the degradation of NO,15 methanol,16 4-chlorophenol,17 or dichloroacetic acid (DCA).18 For these reactions, the number of photons needed for complete decomposition into water and carbon dioxide is well-known, meeting the requirements for a reliable photonic efficiency calculation. For both reaction classes, only the oxidation of the compounds is of interest. It can occur via direct oxidation at the catalysts surface Received: September 10, 2010 Revised: January 25, 2011 Published: April 06, 2011 8014

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Figure 1. Mechanism of deethylation using the example of rhodamine B (without information about energy levels).

or via hydroxyl radicals formed by photogenerated holes. Moreover, molecular oxygen is introduced as electron scavenger to use the reduction potential of electrons for oxidation reactions providing photogenerated oxidants, for example, hydroxyl radicals.19 In the photodegradation of rhodamine B, two different mechanisms are involved, the direct degradation of the chromophoric system and the successive deethylation of the four ethyl groups. Both mechanisms are independent and can proceed side by side. Direct degradation is usually supported by conventional photocatalysis involving electron hole generation by a photocatalyst. On the other hand, the deethylation process is characterized by indirect photocatalysis. Whereas the former process is not linked to special conditions, some requirements must be met to permit deethylation. First, the dye, that is, one electron of the chromophoric system, has to be excited by visible light irradiation. Afterward, electron injection into the conduction band of the semiconductor has to be permitted. Therefore, there must be a direct chemical contact between the catalyst surface and the dye molecule. More precisely, this chemisorption has to be established via an amino group of the dye providing a free electron pair.20 Figure 1 illustrates the proposed mechanism of deethylation.21 Following the dye excitation, excess negative charge inside the conduction band is removed by molecular oxygen forming superoxide radicals, while a proton from the dye molecule is separated generating a double bond. Repeated electron injection generates acetaldehyde and completes the deethylation step. Thereafter, a lone electron pair is again available at the deethylated amino group, permitting subsequent deethylation. Wu et al.22 showed that mineralization of rhodamine B can be facilitated by the deethylation process under visible light as well. Hyperoxide ions can form photogenerated oxidants cleaving the chromophoric system, even if electron hole pair generation by a photocatalyst is not possible (indirect photocatalysis). Li et al.23,24 reported on the degradation of rhodamine B over Pb3Nb4O13 supported on fumed silica and concluded that both reaction mechanisms described above are involved in the degradation of rhodamine B. No deethylation is observed using unsupported lead niobate as photocatalyst. When fumed silica was part of the catalyst, deethylation was observed using monochromatic irradiation at 540 nm as well as at 360 nm. In the first case, a RhBþ 3 radical cation is generated by dye excitation and subsequent electron injection (indirect photocatalysis). In the latter case, deethylation occurs as a consequence of the RhB oxidation by valence band holes generated by UV excitation of the catalyst. In contrast to Li et al., who focused on degradation results and reaction pathways, the present research emphasizes

the pH-based regulation of the deethylation process considering special interactions between the dye and the catalyst surface.

’ EXPERIMENTAL SECTION Catalyst Preparation. The pure Pb3Nb4O13 as well as the silica Aerosil supported lead niobate photocatalysts were prepared by chemical coprecipitation by modifying a procedure reported by Yoshikawa.25 A niobium precursor solution was prepared by dissolving 1.80 g of NbCl5 in 100 mL of 0.1 M nitric acid under inert nitrogen gas. After 30 min of stirring, 40 mL of a 2 M ammonia solution was added. The precipitated hydrated niobium oxide was separated by filtration, thoroughly washed with distilled water, and redissolved in 125 mL of 0.5 M nitric acid and 3 mL of 30% hydrogen peroxide. After treatment in an ultrasonic bath for 60 min, the suspension was stirred for an additional 12 h. The obtained clear niobium precursor solution was of pale yellow color. Subsequently, 1.65 g of Pb(NO3)2 was added, and the niobiumlead precursor solution was hydrolyzed by insertion in 30 mL of 28% ammonia aqueous solution under stirring. For the synthesis of the supported materials, the required amount of silica Aerosil was added to the ammonia solution before hydrolysis. The red-orange colored precipitates were separated by filtration, washed with distilled water, dried at 353 K for 12 h, and finally the precursor powders were calcined at 873 K for 60 min (heating ramp: 1 K/min).24 The samples synthesized by coprecipitation are called Pb3Nb4O13 CP. The silica containing samples are denoted as Pb3Nb4O13 Aerosil x/y, whereas x/y specifies the weight percentages of Pb3Nb4O13/Aerosil. Characterization. Specific surface areas of the samples were measured by nitrogen adsorption employing a Quantachrome Autosorb 3B apparatus using the BET model. Prior to the adsorption measurements, the samples were outgassed at 473 K for 24 h. The crystal structures of the samples were determined by X-ray powder diffraction at room temperature with Cu KR radiation on a Philips X’Pert diffractometer in the 2θ range of 1575. Average particle sizes were calculated from peak broadening of XRD patterns using the Scherrer equation.26 Particle morphologies were examined by means of a JEOL JSM-6700F field-emission scanning electron microscope. Distribution of crystalline catalyst particles in the amorphous silica support was investigated using a JEOL JEM-2100F UHR transmission electron microscope in darkfield mode. UVvis diffuse reflectance spectra were measured on a Varian Cary 4000 UVvis spectrophotometer. Band gap energies were calculated by analysis of the Tauc plots resulting from KubelkaMunk transformation of diffuse reflectance spectra. 8015

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Scheme 1. Corner-Sharing Network of NbO6 Octahedra in the Cubic Pyrochlore Structure of Pb3Nb4O13

Figure 2. XRD patterns of the lead niobates prepared by CP in the absence or presence of silica Aerosil support.

ζ-Potential Measurements. Surface charges of the catalyst

particles were determined via zeta potential measurements using a Malvern Zeta Sizer. The experiments were performed in 1 mL cuvettes. Catalyst suspensions with a concentration of 0.42 g/L catalyst and 1 g/L potassium nitrate were prepared at different pH values. Adjustment of the pH value was achieved by addition of diluted solutions of hydrochloric acid or sodium hydroxide. Rhodamine B Adsorption Studies. Adsorption studies of rhodamine B on Pb3Nb4O13 Aerosil 70/30 were carried out in 10 mL centrifuge glasses. Rhodamine B concentrations were varied from 5 to 250 mg/L, and the pH value was adjusted to 3 or 4.7 to imitate conditions, which are close to those in the photocatalytic process. After being mixed, the suspensions were automatically shaken for 12 h and thereafter centrifuged. Rhodamine B adsorption was quantified by UV/vis spectroscopy. Photocatalytic Tests. Determination of the photocatalytic activity regarding rhodamine B degradation was carried out in a 150 mL Duran glass reactor equipped with quartz glass windows. For irradiation, an Osram XBO 450 W xenon arc lamp equipped with a 10 cm water bath filter to intercept photons of the IR region and hence prevent heat effects was used. In a typical run, 0.12 g of potassium nitrate (10 mmol/L) was added to 120 mL of an aqueous rhodamine B solution with a concentration of ca. 10 mg/L to provide the pH measurement via a glass electrode. After the first sample denoted as Blind was taken, 0.05 g of the catalyst powder (0.42 g/L) was suspended in the dye solution, and the pH was adjusted to a value of 3, if necessary. The suspension was stirred in the dark for 60 min to establish an adsorptiondesorption equilibrium. In the dark as well as under irradiation, air was passed into the suspension to provide molecular oxygen as electron scavenger. Taking the second sample, Ads60/t0 was followed by turning on the light source. Adsorption properties of the different catalysts were calculated using the absorbance changes between the samples Blind and Ads60/t0. The photocatalytic reaction was performed for 360 min. Samples of 2 mL were taken from the reaction suspension at selected irradiation time intervals to gather adequate information about the reaction progress. After separation of the catalyst powder with a centrifuge, the rhodamine B

concentrations were determined by measuring the UVvis absorbance at the maxima, that is, at 554 and 557 nm for pH 4.7 and pH 3, respectively. Potassium Ferrioxalate Actinometry. The photon flux of the Osram XBO 450 W xenon arc lamp under full arc irradiation was quantified by using the standard potassium ferrioxalate actinometer published by Hatchard and Parker.27 Measurements were carried out under argon atmosphere in the same 150 mL Duran glass reactor used for the photocatalytic tests. A 0.02 M potassium ferrioxalate solution was prepared by mixing selfsynthesized and twice recrystallized K3Fe(C2O4) 3 3H2O with 0.05 M sulphuric acid under dark conditions. The actinometer solution used here is capable of absorbing photons up to a wavelength of approximately 450 nm completely. Irradiation was performed over a period of 120 s. For the 0.02 M potassium ferrioxalate actinometer solution, a quantum efficiency of 1.15 was supposed. To isolate the photon flux of the xenon arc lamp from that of ambient light sources, a run without xenon arc lamp was necessary. Photonic efficiencies were determined by dividing the number of degraded reactant molecules, which corresponds to the product of the initial velocity of reaction (v0) and the reaction volume (V), by the photon flux: ζ ð%Þ ¼

v0 3 V 100 photon flux 3

ð1Þ

’ RESULTS AND DISCUSSION Sample Characterization. Lead niobate Pb3Nb4O13 crystallizes in the cubic pyrochlore structure with space group Fd3m. Contrary to the ideal pyrochlore composition A2B2O6Y, onequarter of the A-sites occupied by lead and one-half of the Y-sites taken by oxygen are vacant. Scheme 1 shows the network of corner-sharing NbO6 octahedra, which is considered as a precondition for high photocatalytic activity.28 While niobium is 6-fold coordinated by oxygen, lead is located in the center of a puckered hexagonal pyramid. Figure 2 reveals the XRD patterns of the investigated lead niobates. All in all, the observed XRD patterns fit almost perfectly 8016



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Figure 3. SEM micrographs of (a) Pb3Nb4O13 CP600 and (b) Pb3Nb4O13 Aerosil 70/30, both prepared by coprecipitation and calcined at 873 K for 1 h. Part (c) shows the TEM micrograph of Pb3Nb4O13 Aerosil 70/30 in dark field mode.

ranging from 5 to 30 nm can be found. Figure 3c shows the dark field TEM image, in which the bright areas can be assigned to crystalline parts of the sample, the lead niobate particles. The Pb3Nb4O13 particles are homogeneously distributed in an amorphous fraction assigned to the silica material. Because of the low bulk density of the silica, the volume part of the catalyst is quite small as compared to the silica support. Energy band gaps of the materials were obtained by Kubelka Munk transformation of the UVvis diffuse reflectance spectra. Close to the absorption edge, the reciprocal optical absorption coefficient R is directly proportional to the band gap energy Ebg. This empirical relation is known as the Tauc plot:29 R 3 hν constðhν Ebg Þn

ð2Þ

Figure 4. Tauc plots (for indirect band gap transition) calculated from the UVvis reflectance spectra of the different lead niobate samples.

The KubelkaMunk function F(R¥) is proportional to the reciprocal optical absorption coefficient R:

with the published JCPDS file (no. 23-0352), and a lattice constant of a = 10.563 Å is obtained. The samples are entirely crystallized. Average particle sizes of lead niobate were calculated by the Scherrer equation and yielded about 40 nm in the case of the coprecipitated material and 15.9 nm for the silica-Aerosil supported material with a silica-loading of 30 wt %. XRD patterns with small intensities, for example, the diffraction peak of (311) at 28.01, are covered by background noise, as silica-Aerosil is mostly amorphous. Lead niobate exhibits a specific surface area of 23.1 m2 g1 measured by N2 adsorption according to the BET method. The specific surface area of pure silica-Aerosil amounts to 205.9 m2 g1 before and 62.6 m2 g1 after sintering at 873 K for 60 min. The composite material consisting of 70 wt % Pb3Nb4O13 and 30 wt % silica possesses a specific surface area of 83.5 m2 g1, which exceeds the values of the calcined silica as well as the calcined lead niobate. It seems that sintering behaviors of both components interact, leading to considerable smaller particle sizes and higher surface area. The reduced particle size of Pb3Nb4O13 is easily detectable by XRD and applying of the Scherrer equation and by TEM in dark field mode (cf., Figure 3c), whereas it is not in the case of the silica particles being mostly amorphous. Figure 3a shows a SEM micrograph of Pb3Nb4O13 CP600. A broad particle size distribution ranging from 20 to 100 nm is observed. Part (b) of the figure exhibits the SEM image of the Aerosil supported lead niobate, in which particles with diameters

ðFðRÞ 3 hνÞ1=n constðhν Ebg Þ

ð3Þ

For semiconducting materials providing an indirect band gap transition such as lead niobate, n is defined to be 2, while it values n = 1/2 for direct band gap semiconductors. Band gap energies of the different Pb3Nb4O13 samples are obtained from the intersection point of the linear part of the plot (F(R) 3 hν)1/2 versus hν with the energy axis as shown in Figure 4. For the coprecipitated material, a band gap energy value of 2.8 eV is found, while the Aerosil supported sample has a band gap energy of 2.9 eV. Li et al.23 obtained a band gap energy of 3.01 eV for coprecipitated lead niobate synthesized employing similar conditions. Because of the lead content in the structure leading to hybridization of fully occupied Pb 6s and O 2p orbitals, the valence band top is shifted to more negative values (vs NHE), resulting in a lower band gap energy enabling, in principle, the activation of the photocatalyst by visible light irradiation. Photodegradation. The progress in the photocatalytic degradation of rhodamine B by Pb3Nb4O13 Aerosil 70/30 at pH 3 and pH 4.7 is presented in Figure 5 by monitoring the temporal changes of the UVvis spectra of the aqueous dye solution. The degradation progress obtained with pure coprecipitated Pb3Nb4O13 as photocatalyst at pH 3 and pH 4.7, which is not shown here, is similar to the degradation with the Aerosil supported catalyst at pH 3. The absorption maximum of rhodamine B is pH-dependent and located at 553 nm at a pH of 4.7, while it 8017



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Figure 5. UVvis spectral changes of the aqueous solutions containing rhodamine B and its deethylated derivates: (a) Pb3Nb4O13 Aerosil 70/30 at pH 3.0, (b) Pb3Nb4O13 Aerosil 70/30 at pH 4.7. Reaction conditions: 120 mL of rhodamine B with initial concentration of 10 mg/L; catalyst concentration, 0.42 g/L; reaction temperature, 298 K; 450 W xenon lamp, full arc irradiation; photon flux, 1.75 106 mol/s.

Figure 6. Hypsochromic shift of the absorbance maxima plotted versus the degradation progress (Abs/Abs0) (a) and versus the irradiation time (b).

undergoes a slight shift to 557 nm, if the pH is lowered to 3.30 For a correct interpretation of the UVvis spectra and a subsequent analysis of degradation rates, it is necessary to consider the absorption properties of the rhodamine reaction intermediates in detail. Deethylation. In Figure 6a, the hypsochromic shift in the UVvis spectra is plotted versus the relative degree of dye degradation (Abs/Abs0), whereas Abs stands for the absorbance and corresponds to the concentration approximately. The rhodamine B absorbance at Ads60/t0, that is, after 60 min of adsorption in the dark, is taken as the initial absorbance Abs0.

The shape of the curve reflects the relation between direct degradation of the chromophoric system and the deethylation process. In the absence of deethylation, the absorption maximum should remain at constant wavelength. However, as each deethylation step leads to a hypsochromic shift of about 15 nm, a blue shift of 60 nm can be attributed to the formation of 4-fold deethylated rhodamine B, that is, rhodamine 110.21 Moreover, it should be noted that the molar extinction coefficient of the entirely deethylated product is about 37% lower than that of rhodamine B31,32 and the extinction coefficients of the various deethylated intermediates are supposed to have values between those of the two rhodamine structures. Therefore, a value of 60 nm for Δλ should be reached at Abs/Abs0 ≈ 0.63, if exclusively the deethylation process proceeds without any degradation. Moreover, it should be considered that the specific UVvis bands are overlapping, if more than one rhodamine derivate exists leading to a peak broadening. These effects are responsible for an improper calculation regarding the concentrations of rhodamine derivates via UVvis spectroscopy. Dye photodegradation on the CP material at pH 4.7 gives rise to a blue shift of 9 nm and a distinct band broadening already at 50% degradation representing an appreciable amount of singly deethylated intermediates, whereas a similar shift is observed at pH 3 only if the decomposition is nearly completed. As in the latter case, the absorption maximum of rhodamine B remains almost at the same wavelength when Aerosil supported lead niobate acts as the photocatalyst at pH 3, evidencing that only a small amount of singly deethylated rhodamine B is formed during the reaction time. At a pH of 4.7, however, the photodegradation with the silica containing catalyst leads to a distinguished hypsochromic shift even at low degrees of degradation. In this case, the stepwise deethylation proceeds effectively in addition to the direct degradation without influencing its efficiency. Figure 6b shows the hypsochromic shift of the absorption maximum during the photocatalytic process plotted versus the reaction time. It is obvious that the deethylation process of the unsupported lead niobate samples is an invariable function of the reaction time, while it is independent of the dye concentration and the pH value, as curve shapes coincide very well. Comparing the CP sample with the silica supported lead niobate, it is obvious that an increase in surface area raises the probability of deethylation. 8018



Figure 7. ζ-Potentials of the lead niobates at different pH values. The ζ-potential corresponds with the surface charge. Experimental conditions: catalyst concentration, 0.42 g/L; temperature, 298 K.

pH-Influence on Dye Structure and Catalyst Surface. However, for the silica supported material, the increase in specific BET area alone cannot explain the tremendously increased tendency to undergo deethylation at pH 4.7. Specific interactions between the dye and the catalyst surface depending upon the configuration of the dye as well as the surface charge of the catalyst must also be taken into consideration. Understanding these interactions requires specific knowledge on the influence of the pH variation on the dye structure as well as on the surface charge of the catalyst. Dissolving rhodamine B salt in pure water, the zwitter ionic form, as illustrated in Figure 8b (vide infra), prevails. Regardless of the used catalyst, the carboxylic acid group exists in the deprotonated state, leading to a pH value of almost 4.7. By decreasing the pH of the solution beyond the pKS2 value, which corresponds to 3.22,33 to a value of 3, the carboxylic group changes mostly to its protonated state. Contrary to amino acids, the protonation of the carboxyl group of rhodamine B is favored, and the amino group is protonated only under very weakly basic conditions with the pKB value being 13.75, because the latter results in a loss of an electron pair being part of the chromophoric system. By measuring the ζ-potential, the surface charge of the catalyst can be determined to estimate the fraction of protonated and deprotonated hydroxyl groups on the surface. The ζ-potential is generally smaller in magnitude than the surface charge Ψ0, which ideally follows the Nernst equation.34 In this case, the ζ-potential should become a linear function of the pH value. Nevertheless, experimental observations show that for most oxides the Nernst equation is not fulfilled.35 Figure 7 shows the ζ-potential of the lead niobate samples prepared here in dependence on the pH value of the suspension. Gradual protonation or deprotonation of surface hydroxide groups by raising or lowering the pH value increases or decreases the ζ-potential, respectively. If the ζ-potential is smaller than 30 mV or higher than þ30 mV, a stable suspension is achieved.36 For the CP sample, such a situation occurs for pH values lower than 4.5 or higher than 9, while for the Aerosil supported material a pH higher than 7 needs to be established. Points of zero proton condition (ZPC) were found at pH values of 7.0 and 2.5 for the CP and Aerosil supported materials, respectively. If Aerosil is part of the sample, the negative surface charge of the silica bestrides the positive charge of the lead niobate, resulting in a very low ZPC value.

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As the photocatalytic degradation of rhodamine B was carried out at pH values of 3.0 and 4.7, ζ-potentials at these conditions are of special interest. At pH 3.0, the CP sample exhibits a rather large positive surface charge, whereas the Aerosil supported lead niobate possesses a negligible negative ζ-potential. The potential changes to a considerable lower value of 20 mV at a pH of 4.7 for the latter material, while the CP sample keeps a positive surface charge. Figure 8 shows the proposed modes of interaction between the dye and the catalyst surface of Aerosil supported Pb3Nb4O13 at pH 3 and pH 4.7. The obtained hypsochromic shifts are directly attributed to the resultant favored adsorption modes. A free electron pair at the amino group permitting electron injection is provided by the dye at both pH conditions. Adsorption Model for Coprecipitated Pb3Nb4O13. For the coprecipitated lead niobate, high positive ζ-potential values result in positive surface charges at pH 3 and pH 4.7, leading to strong repulsive interactions between the catalyst surface and the partially positively charged amino groups. At a pH value of 4.7, attractive interaction between the deprotonated carboxyl group and the slightly protonated surface of the catalyst is achieved. Although the carboxyl group is predominantly protonated at pH 3, it still represents the most feasible adsorption site of rhodamine B. The deethylation will be strongly suppressed at both pH values, which coinsides with negligible hypsochromic shifts in the UV/vis spectra. Adsorption Model for Pb3Nb4O13 SiO2-Aerosil. In Aerosil/ Pb3Nb4O13 composite material, the lead niobate is surrounded by amorphous silica particles. At pH 3 and pH 4.7, the latter possesses a considerable negative ζ-potential, which exceeds the positive surface charge of the Pb3Nb4O13 particles. The increased number of deprotonated surface hydroxyl groups at the silica surface changes the adsorption mode of rhodamine B. Although only a few deprotonated hydroxyl groups are available on the silica surface at pH 3, attractive interactions between the silica surface and the positively charged amino groups of the dye exist (Figure 8a). The positively charged lead niobate surface interacts more attractively with the uncharged carboxylic acid than with the positively charged amino group. It is well-known that carboxylic acid groups effectively adsorb on oxide semiconductor surfaces. However, the benzene ring linked to the carboxylic acid function of rhodamine B is not part of, but twisted against, the chromophoric system being excitable by visible light irradiation. This renders electron injection via the carboxylic acid group impossible, leading to an extensive suppression of the deethylation process, which can as well be deduced from the degradation plot (Figure 5). Only if the dye is adsorbed via both amino groups can deethylation proceed. At pH 3, this adsorption is possible but unlikely. Deethylation is consequently found only as the minor process. At pH 4.7, the situation changes as the number of deprotonated hydroxyl groups at the silica surface as well as the deprotonation of the rhodamine B carboxylic acid groups arise (Figure 8b). Thus, attractive interaction is established between both the amino groups of the dye and the surfaces of silica and lead niobate particles, while surrounding silica particles, which are negatively charged, prevent dye adsorption on the lead niobate via the deprotonated carboxyl group. The particular favorable interaction between the catalyst surface and the amino groups of the dye at pH 4.7 will enable electron injection from the dye into the valence band of lead niobate, resulting in distinct deethylation steps, eventually leading to the formation of the 8019



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Figure 8. Adsorption schemes for rhodamine B on the catalyst surface and related ζ-potential values for lead niobate deposited on a silica Aerosil support at pH 3 (a) and pH 4.7 (b).

The adsorption behavior of the composite material Pb3Nb4O13 Aerosil 70/30 toward rhodamine B can be described by the Langmuir adsorption isotherm model. This assumes that all surface adsorption sites are equivalent and that adsorption takes place in a monolayer solely resulting in a saturation of the adsorption value at higher adsorbate concentrations. Although the Langmuir theory was intrinsically developed for gas sorption on ideal metal surfaces, it has been successfully used for studying dye adsorption on various adsorbents. The Langmuir isotherm is described by the following equations (4b, linear form): qeq ¼

K L 3 qmax 3 Ceq 1 þ K L 3 Ceq

Ceq 1 1 ¼ Ceq þ qmax 3 K L 3 qmax qeq

Figure 9. Experimental data and calculated Langmuir isotherms of rhodamine B adsorption on Pb3Nb4O13 Aerosil 70/30 composite material (a) and resulting linearization (b).

4-fold deethylated rhodamine 110. Besides the presented special adsorption mode leading to deethylation via electron injection, it would be possible that the deethylation is caused by a typical photocatalytic reaction as well, if photogenerated oxidants react preferentially with the ethyl groups of rhodamine B. Such a mechanism was reported as being a surface-related reaction, in which photogenerated oxidants formed near adsorption sites oxidize surface near ethyl groups, rather than proceeding in the solution bulk. Therefore, the degree of deethylation should be mainly determined by the adsorption mode of rhodamine B.

ð4aÞ ! ð4bÞ

The Langmuir adsorption constant KL is linked with the adsorption energy, whereas qmax represents the maximal adsorption capacity. Plotting Ceq/qeq versus the rhodamine B concentration (Ceq) at equilibrium conditions should thus result in a linear dependence. Figure 9a shows the plot of the Langmuir isotherms in their linear form for rhodamine B adsorption at pH 3 and pH 4.7. The R2 coefficients, documenting the quality of the fit, amount to 0.998 and 0.996 for adsorption at pH 3 and 4.7, respectively. Because of good linear fits, the calculated Langmuir isotherms match very well with our experimental adsorption data, as can be deduced from Figure 9b. The adsorption capacity of Pb3Nb4O13 Aerosil 70/30 is strongly dependent upon the chosen pH value and rises with decreasing proton concentration. Maximum adsorption capacities (qmax) of 6.96 and 16.01 mg of rhodamine B per gram catalyst are obtained at pH 3 and pH 4.7, respectively. To explain the pH dependency of the adsorption capacities, the model presented in Figure 8 should be taken into consideration. At pH 3, the carboxyl group of rhodamine B forms strong hydrogen bonds with the hydroxyl groups of the catalyst surface, while strong Coulombic interaction is present between the positively charged amino group and the surface (cf., Figure 8a). Because the aromatic systems of the rhodamine B molecule are twisted against one another and a strong interaction toward the surface exists, the dye molecule should be arranged parallel to the 8020



Figure 10. Progress of the rhodamine B concentration obtained from absorbance maxima versus irradiation time (a) and replotted in ln(Abs/ Abs0) versus t (b). Reaction conditions: 120 mL of rhodamine B with initial concentration of ca. 10 mg/L, 450 W xenon lamp, full arc irradiation.

surface. At pH 4.7, the strong chargecharge interaction between the charged amino group and the surface persists, while the hydrogen bond present between the uncharged amino group and the surface will be considerably weaker than the hydrogen bond formed by the carboxylic acid (cf., Figure 8b). This results in weakened interaction between the catalyst surface and the dye. Furthermore, rhodamine B is located in a position that should almost be perpendicular to the surface and hence requires less space at the catalyst surface, leading to the higher observed adsorption capacity at pH 4.7 (cf., Figure 9). Photonic Efficiencies. The xenon arc lamp affords a photon flux of 1.75 106 mol/s at full arc irradiation. As the applied analytical methods are neither suitable for the identification nor for the quantification of intermediates or fragments generated during the photocatalytic process, the average number of photons dissipated for the entire or for any kind of fragmentary degradation of the rhodamine B molecules could not be determined. Therefore, photonic efficiencies have been calculated assuming that the degradation of rhodamine B includes only one measurable step, the transition from colored to colorless. This

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reaction step characterizes the cleavage of the chromophoric system and can be achieved by only one hydroxyl radical; that is, one electronhole pair generated by only one photon is necessary, leading most likely to an underestimation of the real photonic efficiencies. The degradation of rhodamine B is plotted as a function of the illumination time in Figure 10a. Aerosil supported lead niobate features the highest activity among the samples tested here and leads to an entire decolorization in almost 120 min of irradiation regardless of the pH value. Figure 10b shows the progress of the rhodamine absorbance during the photocatalytic process plotted as ln(Abs/Abs0) versus t. It is obvious that the photodegradation of rhodamine B does not follow pseudofirst-order reaction kinetics perfectly, in particular, whenever the deethylation process becomes important. As for all samples, deethylation cannot be totally avoided and will occur parallel with the direct degradation, a separated study of the latter process is impossible, although it is the only relevant process for the determination of a realistic value for the reaction rate. The apparent uncertainty of the dye concentration measurements serves as an explanation for the insufficient correlation that is especially observed for the Aerosil supported samples (cf., Figure 6), while it affects the unsupported samples to a lesser extend as deethylation is rather insignificant in this case. If the pH value is adjusted to 3, the photonic efficiency of the CP material is tripled from 0.48 102% to 1.53 102%. It can be assumed that a higher proton concentration in solution either leads to a better solubility or to an improved adsorption capability of molecular oxygen, thus giving rise to a higher concentration of photogenerated oxidants enhancing the photocatalytic activity. On the other hand, lowering the pH value could lead to the formation of different oxidative species possessing higher oxidation potentials. As deethylation is observed for pure Pb3Nb4O13 neither at pH 3 nor at pH 4.7, the possibility of different oxidative species having an effect on the deethylation efficiency is considered to be unlikely. Lead niobate supported with silica Aerosil results in enhanced photonic efficiencies of 7.50 102% and 8.29 102% at pH values of 4.7 and 3, respectively. Improving the adsorption capacity by addition of silica Aerosil raises the probability for photogenerated oxidants to reach the chromophoric system of rhodamine B. This effect is considered to have a more pronounced influence on enhancing the activity of direct degradation than increasing the proton concentration. Therefore, it can be concluded that pH-dependent oxidative species may influence the direct degradation of rhodamine B in the solution bulk, whereas the deethylation mechanism is unaffected because of being primarily a surface-related process. Moreover, Pb3Nb4O13 Aerosil 70/30 was tested for its photocatalytic activity regarding rhodamine B degradation under near UV and visible light irradiation at pH 4.7. Applying a 360 nm cut off filter (Schott WG360) decreases the photon flux to 66.4% (1.16 106 mol/s) of the initial value, while the reaction rate is reduced to 61.5% (4.0 104 1/s). Consequently, the photonic efficiency remains almost at the same level (7.32 102%). For comparison, the TiO2-based commercial photocatalyst Sachtleben Hombikat UV100 was tested for its activity under full arc irradiation at a pH value of 3. Although the photonic efficiency of 3.11 102% is higher than that of the coprecipitated lead niobate, it reaches only 37.5% of the value achieved with the silica supported catalyst. To verify the photocatalytic role of lead niobate, also a reference measurement with pure Aerosil was performed. Based upon the observed negligible 8021



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by adsorption properties in combination with smaller particle sizes of Pb3Nb4O13 in the silica supported case. The activity enhancing effect of increased surface area can be easily attributed to a higher concentration of active surface sites. If the catalyst is supported on Aerosil, the degradation rates are improved considerably, whereas the pH effect on activity becomes negligible. The activity enhancing effect of the silica Aerosil is attributed to smaller distances between photogenerated oxidants and dye molecules adsorbed on the silica surface.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Figure 11. UV spectral changes observed upon illumination of aqueous solutions containing rhodamine B and its deethylated derivates. Reaction conditions: Pb3Nb4O13 Aerosil 70/30 as photocatalyst at pH 4.7.

reaction rate, the degradation of rhodamine B by the lead niobate occurs photocatalytically (Figure 10). Figure 11 shows the UV region of the absorption spectra obtained for Pb3Nb4O13 Aerosil 70/30 at pH 4.7 representing the spectra of all measured samples. The degradation of smaller aromatic fragments of rhodamine B, for example, benzoquinone,37 which is attributed to a decrease in absorbance in the UV region of the UVvis spectra, is achieved at all times.

’ CONCLUSIONS The photocatalytic degradation of rhodamine B by lead niobates in the absence or presence of Aerosil silica supports can be classified into two independent degradation mechanisms, the deethylation process and the direct degradation by photogenerated oxidants. A hypsochromic shift in the absorbance spectra characterizes the deethylation process. The deethylation efficiency of Aerosil supported Pb3Nb4O13 is dominated by special interactions between rhodamine B and the surface of the catalyst. As both the catalyst surface charge and the structure of the dye molecule are affected by pH variations, the deethylation process can be controlled by adjusting the interactions between the dye and the catalyst surface. Significant deethylation is only achieved if strong chemisorption of the dye via the amino groups allows electron injection. As the amino groups are partially positively charged, strong interactions only exist if the catalyst possesses a negative surface charge as obtained in Aerosil supported material at high pH values. On the other hand, a high pH value is also mandatory to deprotonate the carboxylic acid function of the dye. Otherwise, once more attractive interaction between the catalyst surface and the protonated carboxylic acid groups dominates a suppression of the deethylation process results. Different adsorption behavior leading to an altered tendency regarding the deethylation at various pH values is confirmed by independent measurements of the adsorption isotherms. When pure lead niobate at any pH is used as the catalyst, the deethylation process is found to be almost entirely suppressed. Photonic efficiencies of the lead niobates regarding the direct photocatalytic degradation mechanism are associated with surface area, pH value, and Aerosil support. The efficiency of direct degradation over pure lead niobate is controlled by the pH value potentially leading to different oxidative species possessing varying oxidation potentials, whereas it is mainly determined

’ ACKNOWLEDGMENT This work was supported by the Deutsche Forschungsgemeinschaft (DFG) (WA 1116/18-1). We thank Florian Waltz (Institut f€ur Anorganische Chemie, Leibniz Universit€at Hannover) for support during the zeta potential measurements. ’ REFERENCES (1) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. Rev. 1995, 95, 69–96. (2) Sato, S. Chem. Phys. Lett. 1986, 123, 126–128. (3) Khan, S.; Al-Shahry, M. Science 2002, 297, 2243–2245. (4) Malik, M. A.; Revaprasadu, N.; O’Brien, P. Chem. Mater. 2001, 13, 913–920. (5) Zhang, L.; Wang, W.; Yang, J.; Chen, Z.; Zhang, W.; Zhou, L.; Liu, S. Appl. Catal., A 2006, 308, 105–110. (6) Zou, Z.; Ye, J.; Arakawa, H. J. Cryst. Growth 2001, 229, 462–466. (7) Kim, H. G.; Hwang, D. W.; Lee, J. S. J. Am. Chem. Soc. 2004, 126, 8912–8913. (8) Otsuka-Yao-Matsuo, Sh.; Omata, T.; Yoshimura, M. J. Alloys Compd. 2004, 376, 262–267. (9) Tang, J.; Zou, Z.; Ye, J. Catal. Lett. 2004, 92, 53–56. (10) Ye, J.; Zou, Z. G.; Oshikiri, M.; Matsushita, A.; Shimoda, M.; Imai, M.; Shishido, T. Chem. Phys. Lett. 2002, 356, 221–226. (11) (a) Fujishima, A.; Honda, K. Nature 1972, 238, 37–38. (b) Kudo, A.; Miseki, Y. Chem. Soc. Rev. 2009, 38, 253–278. (12) (a) Bahnemann, D. Solar Energy 2004, 77, 445–459. (b) Herrmann, J.-M. Catal. Today 1999, 53, 115–129. (13) (a) Wark, M.; Tschirch, J.; Bartels, O.; Bahnemann, D.; Rathousky, J. Microporous Mesoporous Mater. 2005, 84, 247–253. (b) Shan, Z.; Wang, W.; Lin, X.; Ding, H.; Huang, F. J. Solid State Chem. 2008, 181, 1361–1366. (14) Wang, Q.; Chen, C.; Zhao, D.; Ma, W.; Zhao, J. Langmuir 2008, 24, 7338–7345. (15) Bannat, I.; Wessels, K.; Oekermann, T.; Rathousky, J.; Bahnemann, D.; Wark, M. Chem. Mater. 2009, 21, 1645–1653. (16) (a) Ismail, A. A.; Bahnemann, D. W.; Bannat, I.; Wark, M. J. Phys. Chem. C 2009, 113, 7429–7435. (b) Kim, W.-I.; Suh, D. J.; Park, T.-J.; Hong, I.-K. Top. Catal. 2007, 44, 499–505. (17) Theurich, J.; Lindner, M.; Bahnemann, D. W. Langmuir 1996, 12, 6368–6376. (18) Sakthivel, S.; Hidalgo, M. C.; Bahnemann, D. W.; Geissen, S. U.; Murugesan, V.; Vogelpohl, A. Appl. Catal., B 2006, 63, 31–40. (19) Novikov, G. F.; Radychev, N. A. Russ. Chem. Bull. 2007, 56, 890–894. (20) Kanoufi, F.; Zu, Y.; Bard, A. J. J. Phys. Chem. B 2001, 105, 210–216. (21) Chen, F.; Zhao, J.; Hidaka, H. Int. J. Photoenergy 2003, 5, 209–217. 8022



ARTICLE

(22) Wu, T.; Liu, G.; Zhao, J.; Hidaka, H.; Serpone, N. J. Phys. Chem. B 1998, 102, 5845–5851. (23) Li, X.; Kako, T.; Ye, J. Appl. Catal., A 2007, 326, 1–7. (24) Li, X.; Ye, J. J. Phys. Chem. C 2007, 111, 13109–13116. (25) Yoshikawa, Y. J. Eur. Ceram. Soc. 2001, 21, 2041–2045. (26) Langford, J. I.; Wilson, A. J. C. J. Appl. Crystallogr. 1978, 11, 102–113. (27) Hatchard, C. G.; Parker, C. A. Proc. R. Soc. London, Ser. A 1956, 235, 518–536. (28) Abe, R.; Higashi, M.; Sayama, K.; Abe, Y.; Sugihara, H. J. Phys. Chem. B 2006, 110, 2219–2226. (29) (a) Tauc, J.; Grigorovic, R.; Vancu, A. Phys. Status Solidi 1966, 15, 627–637. (b) Bi, L.; Feng, J. Y. J. Lumin. 2006, 121, 95–101. (c) Jayaraj, M. K.; Antony, A.; Ramachandran, M. Bull. Mater. Sci. 2002, 3, 227–230. (30) Ramette, R. W.; Sandell, E. B. J. Am. Chem. Soc. 1956, 78, 4872–4878. (31) Leytus, S. P.; Melhado, L. L.; Mangel, W. F. Biochem. J. 1983, 209, 299–307. (32) Mchedlov-Petrosyan, N. O.; Kholin, Y. V. Russ. J. Appl. Chem. 2004, 77, 414–422. (33) Watarai, H.; Funaki, F. Langmuir 1996, 12, 6717–6720. (34) Stumm, W.; Morgan, J. J. Aquatic Chemistry, 2nd ed.; Wiley: New York, 1981. (35) Hohl, H.; Sigg, L.; Stumm, W. Characterization of Surface Chemical Properties of Oxides in Natural Waters. Particulates in Water; Advances in Chemistry, American Chemical Society: Washington, DC, 1980. (36) Everett, D. H. Basic Principles of Colloid Science; The Royal Society of Chemistry: London, 1988. (37) Guo, Y.; Hu, C.; Jiang, C.; Yang, Y.; Jiang, Sh.; Li, X.; Wang, E. J. Catal. 2003, 217, 141–151.

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pH-Control of the Photocatalytic Degradation Mechanism of

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